Multiwavelength observations of a bright impact flash during the January 2019 total lunar eclipse

Journal: Monthly Notices of the Royal Astronomical Society

Manuscript ID MN-19-0577-MJ.R3

Manuscript type: Main Journal

Date Submitted by the Author: 29-Mar-2019

Complete List of Authors: Madiedo, José; Universidad de Huelva, Facultad de Ciencias ExperimentalesOrtiz, Jose; Instituto de Astrofisica de Andalucia, Solar SystemMorales, Nicolas; Instituto de Astrofisica de Andalucia, Solar SystemSantos Sanz, Pablo; Instituto de Astrofísica de Andalucía-CSIC, Solar System

Keywords: meteorites, meteors, meteoroids < Planetary Systems, Moon < Planetary Systems

MULTIWAVELENGTH OBSERVATIONS OF A

BRIGHT IMPACT FLASH DURING THE JANUARY

2019 TOTAL LUNAR ECLIPSEJosé M. Madiedo1, José L. Ortiz2, Nicolás Morales2, Pablo Santos-Sanz2

1 Facultad de Ciencias Experimentales, Universidad de Huelva. 21071 Huelva (Spain).

2 Instituto de Astrofísica de Andalucía, CSIC, Apt. 3004, Camino Bajo de Huetor 50, 18080 Granada, Spain.

ABSTRACT

We discuss here a lunar impact flash recorded during the total lunar eclipse

that occurred on 2019 January 21, at 4h 41m 38.09 0.01 s UT. This is the

first time ever that an impact flash is unambiguously recorded during a lunar

eclipse and discussed in the scientific literature, and the first time that lunar

impact flash observations in more than two wavelengths are reported. The

impact event was observed by different instruments in the framework of the

MIDAS survey. It was also spotted by casual observers that were taking

images of the eclipse. The flash lasted 0.28 seconds and its peak luminosity

in visible band was equivalent to the brightness of a mag. 4.2 star. The

projectile hit the Moon at the coordinates 29.2 0.3 ºS, 67.5 0.4 ºW. In

this work we have investigated the most likely source of the projectile, and

the diameter of the new crater generated by the collision has been

calculated. In addition, the temperature of the lunar impact flash is derived

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from the multiwavelength observations. These indicate that the blackbody

temperature of this flash was of about 5700 K.

KEYWORDS: Meteorites, meteors, meteoroids, Moon.

1. INTRODUCTION

The Earth and the Moon continuously experience the impact of meteoroids

that intercept the path of both celestial bodies. The analysis of these

collisions provides very valuable data that allows us to better understand the

Earth-Moon meteoroid environment. The study of meteoroid impacts on the

Moon from the analysis of the brief flashes of light that are generated when

these particles hit the lunar ground at high speeds has proven to be very

useful to investigate this environment. For instance, the analysis of the

frequency of these events can provide information about the impact flux on

Earth (see e.g. Ortiz et al. 2006; Suggs et al. 2014; Madiedo et al. 2014a,

2014b). Also the initial kinetic energy of the projectile, its mass, and the

size of the resulting crater can be obtained. For events produced by large

(cm-sized or larger) particles, one of the main benefits of this technique over

the systems that analyze meteors produced by the interaction of meteoroids

with the atmosphere of our planet is that a single instrument covers a much

larger area on the lunar surface (typically of an order of magnitude of 106

km2) than that monitored in the atmosphere of the Earth by a meteor-

observing station.

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The monitoring of lunar impact flashes by means of telescopes and high-

sensibility cameras dates back to the 1990s. Since the first systematic

observations performed by Ortiz et al. (1999) in this field, different authors

have obtained information about the collision with the lunar surface of

meteoroids from several sources. Thus, flashes associated with impactors

belonging to the sporadic meteoroid background and to different meteoroid

streams have been recorded and described (see for instance Madiedo et al.

2019 for a comprehensive review about this topic). Some synergies have

been found when this method is employed in conjunction with the technique

based on the monitoring and analysis of meteors produced by meteoroids

entering the atmosphere (Madiedo et al. 2015a,b). Even fresh impact craters

associated to observed lunar impact flashes have been also observed by

means of the Lunar Reconnaissance Orbiter (LRO) probe, which is in orbit

around the Moon since 2009 (Robinson et al. 2015, Madiedo and Ortiz

2018, Madiedo et al. 2019). More recently, since 2015, lunar impact flashes

observations simultaneously performed in several spectral bands allowed us

to estimate the temperature of impact plumes (Madiedo and Ortiz 2016;

Madiedo et al. 2018; Bonanos et al. 2018).

Despite its multiple advantages, this technique has also some important

drawbacks, since the results are strongly dependent on the value given to the

luminous efficiency. This parameter is the fraction of the kinetic energy of

the projectile emitted as visible light as a consequence of the collision. The

value of the luminous efficiency is not known with enough accuracy. The

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comparison between the calculated size of fresh craters associated to

observed impact flashes and the experimental size measured by probes

orbiting the Moon can play a fundamental role to better constrain the value

of this efficiency (Ortiz et al. 2015).

Another drawback of this technique is related to the fact that, since most of

these flashes are very dim, they must be recorded against a dark

background. For this reason, the method is based on the monitoring of the

nocturnal region of the Moon. The area directly illuminated by the Sun must

be avoided in order to prevent the negative effects of the excess of scattered

light entering the telescopes. This implies that, weather permitting, the

monitoring by means of telescopes of these flashes is limited to those

periods where the illuminated fraction of the lunar disk ranges between

about 5% and 50-60%, i.e., about 10 days per month during the waxing and

waning phases (Ortiz et al. 2006, Madiedo et al. 2019). Lunar eclipses

provide another opportunity to monitor lunar impact flashes out of this

standard observing period, since during these the Moon gets dark. However,

because of the typical duration of lunar eclipses, this extra observational

window is relatively short when compared to a standard observing session.

Besides, the possibility to detect dimmer impact flashes, which are more

frequent than brighter ones, depend on the intrinsic brightness of the eclipse,

which in turn depend on the aerosol content at stratospheric levels. In

general, the lunar ground is brighter in visible light during a lunar eclipse

than the lunar ground in standard observing periods during the waning and

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waxing phases. These factors, which pose some difficulties to the detection

of lunar impact flashes, might have contributed to the fact that, despite

several researchers have conducted impact flashes monitoring campaigns

during lunar eclipses, no team succeeded until now. The first lunar impact

flash monitoring campaign performed by our team during a total lunar

eclipse was conducted by the second author of this work in October 2004. In

2009, the pioneer survey developed by Ortiz et al. (1999) was renewed and

named Moon Impacts Detection and Analysis System (MIDAS) (Madiedo

et al. 2010; Madiedo et al. 2015a, 2015b). This project is conducted from

three astronomical observatories located in the south of Spain: Sevilla, La

Sagra and La Hita (Madiedo and Ortiz 2018, Madiedo et al. 2019). In this

context, our survey observed a flash on the Moon during the total lunar

eclipse that took place on 2019 January 21. This flash was also spotted by

casual observers that were taking images of this eclipse, or streaming it live

on the Internet

(https://www.reddit.com/r/space/comments/ai79zy/possible_meteor_impact

_on_moon_during_the_eclipse/). The MIDAS survey was the first to

confirm that this flash was generated as a consequence of the collision of a

meteoroid with the lunar soil at high speed, so that this is the first lunar

impact flash ever recorded during a lunar eclipse and discussed in the

scientific literature. The news was covered by communication media all

around the world. From a scientific point of view, it offered the opportunity

to monitor the Moon with an angular orientation very different to that of the

regular campaigns at waxing and waning phases and it was a good

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opportunity to test new equipment for the monitoring of lunar impact

flashes, and provided valuable data in relation to the study of impact

processes on the Moon. We focus here on the analysis of this impact event.

2. OBSERVATIONAL TECHNIQUE

The impact flash discussed in this work was observed from Sevilla on 2019

January 21. Our systems at the observatories of La Sagra and La Hita could

not operate because of adverse weather conditions. In Sevilla, five f/10

Schmidt-Cassegrain telescopes were used. Two of these instruments had an

aperture of 0.36 m, and the other three telescopes had a diameter of 0.28 cm.

These telescopes employed a Watec 902H Ultimate video camera connected

to a GPS-based time inserter to stamp time information on each vide frame.

The configuration of these cameras, which are sensitive in the wavelength

range between, approximately, 400 and 900 nm, is explained in full detail in

Madiedo et al. (2018). The observational setup consisted also of two 0.10 m

f/10 refractors endowed with Sony A7S digital cameras, which provided

colour imagery and employ the IMX235 CMOS sensor. One of these was

configured to take still images each 10 s with a resolution of 4240x2832

pixels, while the other recorded a continuous video sequence of the eclipse

at 50 fps with a resolution of 1920x1080 pixels. A third Sony A7S camera

working in video mode was attached to a Schmidt-Cassegrain telescope

with an aperture of 0.24 m working at f/3.3. However, because of a

technical issue that occurred during the eclipse, this telescope could not be

finally operated. The Sony A7S cameras are sensitive within the wavelength

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range between, approximately, 400 and 700 nm. These have been used in

the framework of our survey for the first time during this monitoring

campaign to take advantage of the colour information they could provide.

Also, the larger field of view of these instruments allowed for a full

coverage of the lunar surface during the totality phase of the eclipse, in

contrast with the Schmidt-Cassegrain telescopes with the Watec cameras,

which can monitor only an area of the Moon of around 4·106 to 8·106 km2

(see for instance Madiedo et al. 2015a,b and Ortiz et al. 2015).

No photometric filter was attached to the cameras employed with the 0.36 m

and two of the 0.28 m Schmidt-Cassegrain telescopes. These provided

images in the wavelength range between, approximately, 400 and 900 nm.

The third 0.28 m SC telescope employed a Johnson-Cousin I filter.

Observations performed with the two refractors were also unfiltered.

We did not focus on the monitoring of any particular region on the lunar

disk. Instead, our telescopes were aimed so that the whole lunar disk was

monitored during the totality phase of the eclipse, with each instrument

covering a specific area of the lunar surface, and with at least two

instruments monitoring a common area. Before and after the totality, the

region of the Moon not occulted by the Earth's shadow was avoided. The

MIDAS software (Madiedo et al. 2010, 2015a) was employed to

automatically detect lunar impact flashes in the images obtained with the

above-mentioned instrumentation.

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3. OBSERVATIONS

Our lunar monitoring campaign took place on 2019 January 21 from 3h 33m

UT to 6h 50m UT. These times correspond to the first and last contact with

the Earth's umbra, respectively. Excellent weather conditions allowed us to

monitor the Moon during the whole time interval, so the effective observing

time of was of 3.2 hours. This resulted in the detection of a flash at 4h 41m

38.09 0.01 s UT (Figure 1), about 21 seconds after the totality phase of the

eclipse began. This event, which lasted 0.28 s, was simultaneously recorded

by two of our instruments: one of the 0.36 m Schmidt-Cassegrain

telescopes, and the 0.1 m refractor with the Sony A7S camera that recorded

the continuous video sequence of the eclipse. This flash was also reported in

social networks by several observers at different locations in Europe,

America and Africa

(https://www.reddit.com/r/space/comments/ai79zy/possible_meteor_impact

_on_moon_during_the_eclipse/). The MIDAS team confirmed that it was

associated with an impact event on the Moon. Table 1 contains the main

parameters derived for this impact flash. By means of the MIDAS software

(Madiedo et al. 2015a, 2015b) we determined that the impactor hit the

Moon at the selenographic coordinates 29.2 0.3 ºS, 67.5 0.4 ºW, a

position close to crater Lagrange H. This is located next to the west-south-

west portion of the lunar limb.

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It is worth mentioning that astronomers at the Royal Observatory in

Greenwich reported a second flash at 4:43:44 UT (Emily Drabek-Maunder,

personal communication). We tried to locate this flash in our recordings by

checking them automatically with our MIDAS software. We also checked

them manually, by performing a visual inspection of the videos frame by

frame. We allowed for a timing uncertainty of around 1 minute, which is

well above the 5 seconds time difference between the time reported by this

observatory for the first flash (4:41:43 UT) and the time specified by our

GPS time inserters. However, this event was not present in any of the

images recorded by our systems and, to our knowledge, no other casual

observer spotted it. This means that it should have been produced by a

different phenomenon, and not by a meteoroid hitting the lunar ground. The

MIDAS survey uses at least two instruments monitoring the same lunar area

in order to have redundant detection to discard false positive impact flashes

due to cosmic ray hits, satellite glints and other possible phenomena that

may mimic the impact flashes.

4. RESULTS AND DISCUSSION

4.1. Impactor source

Since the technique employed to detect lunar impact flashes cannot

unambiguously provide the source of the impactors that produce these

events (Madiedo et al. 2015a, 2015b, 2019), we have followed the approach

described in (Madiedo et al. 2015a, 2015b) to determine the most likely

source of the meteoroid that generated the flash discussed here.

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The observing date did not coincide with the activity period of any major

meteor shower on our planet and so the impactor should be associated either

with a minor meteoroid stream or with the sporadic meteoroid component.

Our meteor stations, which operate in the framework of the SMART project

(Madiedo 2014, 2017), recorded that night meteors from the January Comae

Berenicids (JCO), the -Cancrids (DCA), and the -Geminids (RGE), but

the activity of all of these corresponded to a zenithal hourly rate (ZHR) < 1

meteor/h. Besides, the geometry for the impact of the DCA and RGE

streams did not fit that of the lunar impact flash: these meteoroids could not

hit the lunar region where the flash was recorded. So, we considered the

sporadic background and the JCO meteoroid stream as potential sources of

the event. The association probabilities corresponding to these sources,

labelled as pSPO and pJCO, respectively, were obtained by following the

technique developed by Madiedo et al. (2015a, 2015b). Thus we have

calculated pJCO with our software MIDAS, which obtains this probability

from Equation (15) in the paper by Madiedo et al. (2015b). In this

calculation the zenithal hourly rate and the population index of the January

Comae Berenicids have been set to 1 meteor/h and 3, respectively, and

HR=10 meteors/h was set for the activity of the sporadic component (see for

instance Dubietis and Arlt, 2010). From this analysis pJCO yields 0.01, with

pSPO = 1 - pJCO

= 0.99. According to this, the probability that the impactor is

linked to the sporadic meteoroid component is of about 99%. In these

calculations an average impact velocity and an impact angle of sporadics on

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the Moon of 17 km s-1 and 45º, respectively, have been assumed (Ortiz et al.

1999). For impactors associated with the JCO meteoroid stream this velocity

was set to 65 km s-1 (see e.g. Jenniskens 2006) and, according to the impact

geometry, the angle of impact would be of around 54º in this case.

4.2. Impactor kinetic energy and mass

We recorded the impact flash with the Watec camera in white light only.

Since no observations with different photometric filters were available for

this CCD device, we could not employ color terms for the photometric

analysis of the event. As explained in the next section, color terms could be

employed in the case of the Sony A7S camera. So, as in previous works

(see, e.g., Ortiz et al. 2000, Yanagisawa et al. 2006, Madiedo et al. 2014),

the brightness of the flash as recorded with the Watec camera was estimated

by comparing the luminosity of this event with the known V magnitude of

reference stars observed with the same instrumentation at equal airmass. In

this way we could determine that the peak magnitude of the impact flash

was 4.2 0.2. Figure 2 shows the lightcurve of the flash as recorded by

means of the 0.36 m telescope that spotted the event. Using t=0.28 in the

empiric equation

t=2.10exp(-0.46±0.10m) (1)

that links impact flash duration t and magnitude m (Bouley et al. 2012), we

come up with a 4.1 mag for the flash, which is close to the derived 4.2 mag.

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The calculations in this section are performed from the data collected by this

instrument, since its larger aperture and the higher sensitivity of its CCD

camera allowed us to record the evolution of the impact flash in much more

detail than with the 0.1 m refractor. This refractor telescope just registered

the peak luminosity of the flash and so the lightcurve of the event cannot be

constructed from its recordings.

As explained in detail in Madiedo et al. (2018), the energy radiated on the

Moon by the flash can be obtained from the integration of the power

radiated by the event:

(2)2)5.2/m(8 Rf10·10·75.3P

Here the magnitude of the flash varies with time according to the lightcurve

of the event, and f quantifies the degree of isotropy of the emission of light.

Since we have considered that light was isotropically emitted from the lunar

ground, we have set f = 2 (Madiedo et al. 2018). The distance between our

observatory on Earth and the impact location on the Moon at the instant

when the event took place was R= 364831.2 km. For the wavelength range

Δλ corresponding to the luminous range we have set = 0.5 μm (see for

instance Ortiz et al. 2000 and Madiedo et al. 2019).. By entering these

parameters in Eq. (2) the energy radiated on the Moon yields E =

(1.960.39)·107 J.

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This radiated energy is a fraction of the kinetic energy Ek of the meteoroid.

That fraction is called the luminous efficiency, which is wavelength-

dependent and is usually denoted by (Bellot Rubio et al. 2000a, 2000b;

Ortiz et al. 2000; Madiedo et al. 2018, 2019):

E = Ek (3)

Since the value of the radiated energy derived from Eq. (2) depends on the

wavelength range considered, the luminous efficiency for that same spectral

range defined Δλ by must be employed. On the contrary, we would arrive to

the non-sense conclusion that the kinetic energy of the projectile would be

also a function of the spectral range, instead of depending only on the mass

and velocity of the projectile. The concept "luminous" refers to the above-

mentioned luminous range, and it was defined to correspond to the range of

sensitivity of typical CCD detectors (i.e., from around 400 to about 900 nm)

used in the first works on lunar impact flashes and luminous efficiencies

(see e.g. Bellot-Rubio et al. 2000a, 2000b; Ortiz et al. 2000; Yanagisawa et

al. 2006). Other wavelength ranges can be of course defined and employed,

but this consistency between Δλ, E and must be maintained. For other

spectral ranges the fraction of the kinetic energy of the impacting meteoroid

converted into radiation in the corresponding photometric bands should be

denoted by using subscripts, such as R, for the R-band, I for the I-band,

etc., to avoid confusing it with (Madiedo et al. 2018, 2019). In previous

works the value employed for the luminous efficiency was =2·10-3 (Ortiz

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et al. 2006, 2015). However, this value was derived by assuming f=3 for the

degree of isotropy factor (see, for instance, Ortiz et al. 2006). Since in this

work we have considered f=2, we have to multiply this value of the

efficiency by 3/2, as explained in Madiedo et al. (2018). As a consequence

of this, the value considered for η in the luminous range for the flash yields

η = 3·10-3. In this way, the kinetic energy Ek of the impactor is Ek =

(6.550.63)·109 J. The impactor mass M derived from this kinetic energy is

M = 45 8 kg for a sporadic meteoroid impacting at velocity of 17 km s-1.

Its size is readily obtained from the bulk density of the particle. The average

value of this bulk density for projectiles associated with the sporadic

meteoroid background is P=1.8 g cm-3 according to Babadzhanov and

Kokhirova (2009). This density yields a diameter for the impactor DP = 36

2 cm. However, if the projectile consisted of soft cometary materials, with a

bulk density of 0.3 g cm-3, or ordinary chondritic materials, with P = 3.7 g

cm-3 (Babadzhanov and Kokhirova 2009), the size of the projectile would

yield DP = 66 4 cm and DP = 29 2 cm, respectively.

4.3. Temperature of the impact plume

Unfortunately, the impact flash was not recorded by the 0.28 m telescope

with the Johnson I filter, since the event took place outside the field of view

of this instrument. So, we could not derive the temperature of the impact

flash by comparing the energy flux density measured in the luminous and

the I ranges (Madiedo et al. 2018). Instead, we followed here a different

approach on the basis of the colour images recorded by the 0.1 m refractor

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and the Sony A7S camera. The decomposition of these colour images into

its individual R, G and B channels (Figure 3) provides a multiwavelength

observation of the impact flash, which can be employed, for instance to

derive the flash temperature, assuming blackbody emission. To do so, we

have performed a photometric calibration of the Sony A7S camera to derive

the flash magnitude in the Johnson-Cousins R, V and B bands from its

measured luminosity in R, G and B channels of the video stream. For this

conversion color term corrections are necessary. It is worth mentioning

that the Sony A7S camera has a built-in NIR blocking filter, but in the

spectral response of the device, no leakage in the NIR was observed. The

calibration procedure has been performed as follows.

The magnitudes mR, mV and mB in the Johnson-Cousins photometric system

are given by the following standard relationships:

mR = r + ZPR + (mV-mR) CR - KR A (4)

mV = v + ZPV + (mV-mR) CV - KV A (5)

mB = b + ZPB + (mB-mV) CB - KB A (6)

In these equations ZPR, ZPV, and ZPB are the corresponding zero points for

each photometric band, KR, KV, and KB are the extinction coefficients, and

A is the airmass; r, v, and b are the instrumental magnitudes in R, V and B

band, and are defined by

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r = -2.5log(SR) (7)

v = -2.5log(SG) (8)

b = -2.5log(SB) (9)

where SR, SG, and SB are the measured signals. We employed 30 calibration

stars within the Messier 67 open cluster, with known mR, mV and mB, to

obtain the value of the color terms CR, CV, and CB and the coefficients ZPR,

ZPV, ZPB, KRA, KVA, and KBA by performing a least-squares fit (Figures 4

to 6). These stars were observed with the same refractor telescope and Sony

A7S camera employed to record the flash. Their signals SR, SG and SB were

measured by performing an aperture photometry. Since the calibration stars

and the impact flash were observed at the same airmass, the least-squares fit

provided the sum of ZP and KA in a single constant for each band R, V and

B. The values resulting from this fit are shown in Table 2. By inserting in

Eqs (4-6) the measured flash signals in R, G and B channels, the peak

magnitude of the flash in R, V and B bands yield, respectively, mR= 3.53

0.19, mV= 4.08 0.10 and mB= 4.75 0.09. The value calculated for mV fits

fairly well the 4.2 0.2 magnitude in V band derived from the images

obtained with the Watec camera.

From these magnitudes, the energy flux densities observed on our planet for

the above-mentioned bands (labelled as FR, FV, and FB) have been estimated

by employing the following equations:

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(10))5.2/m(8R

R10·10·80.1F

(11))5.2/m(8V

V10·10·75.3F

(12))5.2/m(8B

B10·10·70.6F

where the multiplicative constants 1.80·10-8, 3.75·10-8 and 6.70·10-8

correspond to the irradiances, in Wm-2μm-1, for a mag. 0 star in the

corresponding photometric band. The effective wavelengths for these bands

are R = 0.70 m, V = 0.55 m, and B = 0.43 m, respectively. These

parameters have been provided by the magnitude to flux converter tool of

the Spitzer Science Center

(http://ssc.spitzer.caltech.edu/warmmission/propkit/pet/magtojy/). The flux

densities given by Eqs (10-12) are plotted in Figure 7. By assuming that the

flash behaves as a blackbody, these flux densities have been fitted to

Planck's radiation law. The best fit is obtained for T = 5700 300 K. This

temperature agrees with the statistics of flash temperatures derived with 2-

color measurements from the Neliota survey, for which blackbody

temperatures ranging between 1300 and 5800 K have been estimated

(Avdellidou and Vaubaillon 2019). Our result is in the high-end tail of the

blackbody temperature flash distribution shown in Avdellidou and

Vaubaillon (2019) from a sample of 55 impact flashes with magnitudes in R

band ranging between 6.67 to 11.80. Lower temperatures can be fit to our

data by assuming optically thin emission modulated by the optical depth,

but we cannot determine the optical depth of the emitting hot cloud at

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different wavelengths without making too many assumptions. When

observations at 4 or more wavelengths become available we will be able to

shed more light on this.

4.4. Crater size and potential observability by lunar spacecraft

The estimation of the size of fresh craters associated with observed lunar

impact flashes is fundamental to allow for a better constraint of the

luminous efficiency, a key parameter which is not yet known with enough

accuracy. Thus, if these craters are later on observed and measured by

probes in orbit around the Moon, the comparison between predicted and

experimental sizes is of a paramount importance to test the validity of the

parameters and theoretical models employed to analyze these impacts.

Different models, which are also called crater-scaling equations, can be

employed to estimate the size of these fresh craters, and most studies in

these field employ either the Gault model or the Holsapple model. The

Gault equation is given by the following relationship (Gault, 1974):

(13) 3/129.0k

5.0t

6/1p sinE25.0D

D is the rim-to-rim diameter, ρp and ρt are the projectile and target bulk

densities, respectively, and the angle of impact θ is measured with respect to

the local horizontal (Melosh, 1989). We have employed θ=45º for sporadic

meteoroids, and for the target bulk density we have considered ρt = 1.6 g

cm-3. By entering in this model the previously-obtained value of the kinetic

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energy Ek, the diameter D for impactor bulk densities ρp of 0.3, 1.8 and 3.7

g cm-3 yields 10.1 ± 0.5 m, 13.6 ± 0.6 m, and 15.3 ± 0.7 m, respectively.

We have also derived the crater size from the following equation, which was

proposed by Holsapple (1993):

(14).6.23/1

t

vr

MKD

D is again the rim-to-rim diameter, and v is an adimensional factor which

has the following form:

(15)

23

22

326

P

t2

t2

326

P

t21v ))sin(V(

YK))sin(V(

agK

with K1=0.2, K2=0.75, Kr=1.1, =0.4, =0.333 and Y = 1000 Pa. The value

of the gravity on the lunar surface is g = 0.162 m s-2; the parameters a, M,

and V are the impactor radius, mass, and impact velocity, respectively. For

meteoroid bulk densities ρp of 0.3, 1.8 and 3.7 g cm-3, Eq. (14) yields for the

rim-to-rim crater diameter D 10.4 ± 0.5 m, 13.3 ± 0.6 m, and 15.8 ± 0.7 m,

respectively, for a sporadic meteoroid hitting the Moon with an average

collision velocity of 17 km s-1.

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Values derived from our analysis of the crater diameter are summarized in

Table 3. Both above-mentioned scaling models predict a similar rim-to-rim

diameter D for the same impactor bulk density, with D ranging from about

10 to 15 m. Because of its small size, this crater cannot be observed by

telescopes from our planet. But probes in orbit around the Moon can spot it,

provided that these can take pre- and post- impact images of the area where

the meteoroid collision takes place. For instance, craters produced by

previous collisions that gave rise to observed impact flashes were

successfully identified by cameras onboard the Lunar Reconnaissance

Orbiter (LRO), which orbits the Moon in a polar orbit since 2009 (Madiedo

et al. 2014, 2019; Suggs et al. 2014, Robinson et al. 2015). These

observations are or a paramount importance, since they would allow us to

compare the actual and predicted crater diameters to check the validity of

our assumptions. This would also provide a better constraint for the

luminous efficiency associated with the collision of meteoroids on the

Moon.

5. CONCLUSIONS

We have focused here on a lunar impact flash recorded during the Moon

eclipse that occurred on 2019 January 21. This is the first impact flash

unambiguously recorded on the Moon during a lunar eclipse and discussed

in the scientific literature. The event, spotted and confirmed in the

framework of the MIDAS survey, was also imaged by casual observers in

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Europe, America and Africa. The peak V magnitude of the flash was 4.2

0.2, and its duration was of 0.28 s. According to our analysis, the most

likely scenario with a probability of 99% is that the impactor that generated

this flash was a sporadic meteoroid. By considering a value for the luminous

efficiency of 3·10-3 and an impact speed of 17 km/s, the estimated mass of

the impactor yields 45 8 kg. By employing the Gault scaling law, the rim-

to-rim diameter of the crater generated during this collision ranges from

10.1 ± 0.5 m (for an impactor bulk density of 0.3 g cm-3) to 15.3 ± 0.7 m

(for a bulk density of 3.7 g cm-3). The Holsapple model predicts a similar

size. The crater could be measured by a probe in orbit around the Moon,

such as for instance the Lunar Reconnaissance Orbiter. The comparison

between the predicted and the experimental crater size could be very

valuable to allow for a better constraint of the luminous efficiency for

meteoroids impacting the lunar ground.

This is also the first time that lunar impact flash observations in more than

two wavelengths are reported. The impact plume blackbody temperature has

been estimated by analyzing the R, G and B channels of the color camera

employed to record the event. This multiwavelength analysis has resulted in

a peak temperature of 5700 300 K.

ACKNOWLEDGEMENTS

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We acknowledge funding from MINECO-FEDER project AYA2015-

68646-P, and also from project J.A. 2012-FQM1776 (Proyectos de

Excelencia Junta de Andalucía).

REFERENCES

Avdellidou C. and Vaubaillon J., 2019, MNRAS, in press.

Babadzhanov P.B. and Kokhirova G.I., 2009, A&A, 495, 353.

Bellot Rubio L.R., Ortiz J.L., Sada P.V., 2000a, ApJ, 542, L65.

Bellot Rubio L.R., Ortiz J.L., Sada P.V., 2000b, Earth Moon Planets, 82–83,

575.

Bonanos A.Z., Avdellidou C., Liakos A., Xilouris, E.M. et al., 2018,

Astronomy & Astrophysics, 612, id.A76.

Bouley S., Baratoux D., Vaubaillon J., Mocquet A., et al., 2012, Icarus, 218,

115.

Dubietis A, Arlt R, 2010, EMP, 106, 105.

Gault D.E., 1974, In: R. Greeley, P.H. Schultz (eds.), A primer in lunar

geology, NASA Ames, Moffet Field, p. 137.

Page 22 of 34

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

Holsapple K.A., 1993, Annu. Rev. Earth Pl. Sc. 21, 333.

Jenniskens P., 2006, Meteor Showers and their Parent Comets. Cambridge

University Press.

Madiedo J. M., 2014, Earth, Planets and Space, 66, 70.

Madiedo J.M., Trigo-Rodríguez J.M., Ortiz J.L., Morales N., 2010,

Advances in Astronomy, doi:10.1155/2010/167494.

Madiedo J.M., Ortiz J.L., Morales N., 2018, MNRAS, 480, 5010.

Madiedo J.M., Ortiz J.L., 2018, MIDAS System. In: Cudnik B. (eds)

Encyclopedia of Lunar Science. Springer, Cham, doi:10.1007/978-3-319-

05546-6_128-1

Madiedo J.M., Ortiz J.L., Morales N., Cabrera-Caño J., 2014a, MNRAS,

439, 2364.

Madiedo J.M., Ortiz J.L., Trigo-Rodríguez J.M., Zamorano J., Konovalova

N., Castro-Tirado A.J., Ocaña F., Sánchez de Miguel A., Izquierdo J.,

Cabrera-Caño J., 2014b, Icarus, 233, 27.

Page 23 of 34

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

Madiedo J.M., Ortiz J.L., Morales N., Cabrera-Caño J., 2015a, PSS,

111,105.

Madiedo J.M., Ortiz J.L., Organero F., Ana-Hernández L., Fonseca F.,

Morales N., Cabrera-Caño J., 2015b, A&A, 577, A118.

Madiedo J.M., Ortiz J.L., 2016, https://www.cosmos.esa.int/documents/

653713/1000954/08_ORAL_Madiedo.pdf/d64b2325-8a37-432a-98f1-

28784da94a40

Madiedo J.M., Ortiz J.L., Yanagisawa, M., 2019, Impact flashes of

meteoroids on the Moon. In: Campbell-Brown M., Ryabova G., Asher, D.

(eds) Meteoroids: Sources of Meteors on Earth and Beyond. Cambridge

University Press, Cambridge, UK, in press.

Melosh H.J., 1989. Impact Cratering: A Geologic Process. Oxford Univ.

Press, New York.

Ortiz J.L., Aceituno F.J., Aceituno J., 1999. A&A, 343, L57.

Ortiz J.L., Sada P.V., Bellot Rubio L.R., Aceituno F.V., Aceituno J.,

Gutierrez P.J., Thiele U., 2000, Nature, 405, 921.

Page 24 of 34

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

Ortiz J.L., Quesada J.A., Aceituno J., Aceituno F.J., Bellot Rubio L.R.,

2002, ApJ, 576, 567.

Ortiz J.L., Aceituno F.J., Quesada J.A., Aceituno J., Fernandez M., et al.,

2006, Icarus, 184, 319.

Ortiz J.L., Madiedo J.M., Morales N., Santos-Sanz P., and Aceituno F.J.,

2015. MNRAS, 454, 344.

Robinson M.S, Boyd A.K., Denevi B.W., Lawrence S.J., McEwen A.S.,

Moser D. E., Povilaitis R.Z., Stelling R.W., Suggs R.M., Thompson S.D.,

Wagner R.V., 2015, Icarus, 252, 229.

Suggs R.M., Moser D.E., Cooke W.J., Suggs R.J., 2014, Icarus 238, 23.

Yanagisawa M., Ohnishi K., Takamura Y., Masuda H., Ida M., Ishida M.,

2006, Icarus, 182, 489.

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TABLES

Date and time 2019 January 21 at 4h 41m 38.09 ± 0.01s UT

Peak brightness (magnitude) 4.2 0.2 in V band

Impact location Lat.: 29.2 0.3 ºS, Lon.: 67.5 0.4 ºW

Duration (s) 0.28

Impactor kinetic energy (J) (6.55 0.63)·109

Impactor mass (kg) 45 8

Table 1. Characteristics of the lunar impact flash analysed here.

ZPR + KRA 10.81 0.06

ZPV + KVA 11.07 0.01

ZPB + KBA 11.71 0.02

CR -0.398 0.11

CV -0.018 0.006

CB 0.157 0.05

Table 2. Results obtained from the photometric calibration of the Sony A7S

camera, as defined by Equations (4 to 6).

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Scaling

law

Impact

angle (º)

Meteoroid

Density

(g cm-3)

Meteoroid

Mass

(kg)

Impact

Velocity

(km s-1)

Crater

Diameter

(m)

Gault 45 0.3 45±8 17 10.1±0.5

Gault 45 1.8 45±8 17 13.6±0.6

Gault 45 3.7 45±8 17 15.3±0.7

Holsapple 45 0.3 45±8 17 10.4±0.5

Holsapple 45 1.8 45±8 17 13.3±0.6

Holsapple 45 3.7 45±8 17 15.8±0.7

Table 3. Diameter of the fresh crater, according to the Gault and the

Holsapple models.

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FIGURES

Figure 1. Lunar impact flash recorded on 2019 January 21 by the 0.36 m SC

(up) and the 0.10 m refractor (down) telescopes.

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Figure 2. Lightcurve (evolution of V-magnitude as a function of time) of the

impact flash recorded by the 0.36 m telescope.

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Figure 3. Decomposed image of the lunar impact flash into the three basic

colour channels R, G, and B, during the peak luminosity of the event.

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Figure 4. Photometric calibration for R band performed by employing 30

reference stars in Messier 67. The solid line corresponds to the best fit

obtained from measured data.

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Figure 5. Photometric calibration for V band performed by employing 30

reference stars in Messier 67. The solid line corresponds to the best fit

obtained from measured data.

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Figure 6. Photometric calibration for B band performed by employing 30

reference stars in Messier 67. The solid line corresponds to the best fit

obtained from measured data.

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Figure 7. Flux densities obtained in R, V, and B bands. The solid line

represents the best fit of these data to the flux emitted by a blackbody at a

temperature T=5700 K.

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